A reciprocating internal combustion engine is well-known which comprises, as illustrated in FIGS. 1-4, a cylinder block 20 having a bore 22, a piston 26 arranged in the cylinder bore 22 for reciprocatory motion therein, and a combustion chamber 24 enclosed by the piston 26, cylinder block 20 and a head 29. A sealing device for providing a seal between the piston 26 and the wall of the cylinder bore 22 includes a piston ring 40 fitted into an annular groove 32 formed in the outer peripheral surface 30 of the piston 26. In the conventional design, both the piston ring 40 and the piston ring groove 32 into which the piston ring 40 is fitted have a generally rectangular cross-sectional shape of uniform thickness.
In recent years, however, alternative non-conventional cross-sectional shapes have been proposed for both the piston ring and for the piston groove, for the purpose of overcoming certain disadvantages encountered when using the conventional piston ring and groove design. For example, Tokoro, in U.S. Pat. No. 4,774,917, discloses piston rings and grooves having somewhat L-shaped cross-sections, with one surface of both the ring and groove inclined with respect to the axis of the piston. This design is intended to reduce friction between the piston ring and the cylinder wall, to prevent the top of the piston from melting from the heat in the combustion chamber, and to reduce blow-by gas leakage past the piston ring during the expansion stroke of the engine. Fujikawa, in U.S. Pat. No. 4,346,685,discloses an L-shaped keystone piston ring and similarly shaped groove in the piston. This design is intended to improve sealing between the piston and the cylinder wall and to reduce the possibility of seizing of the ring and groove surface. Neeme, in U.S. Pat. No. 2,638,390, discloses a piston groove of rectangular cross-section, but having a countergroove or cutback in its bottom, and a plurality of radial outlet canals in the sidewall of the groove opposite the combustion chamber. This design is intended to reduce friction and wear on the piston ring and cylinder wall by providing a path for gas and oil to seep behind and around the pisten ring during operation of the engine. Winston, in U.S. Pat. No. 4,883,029, discloses a piston ring and groove having a generally rectangular cross-sectional shape, except for a projection on the ring which interlocks with a recess in the groove. This design is intended to prevent rotation of the piston ring with respect to the piston.
While these alternative piston ring and groove designs may be somewhat effective in accomplishing the purpose for which they were intended, an inherent disadvantage remains with all the prior art designs. There is a tendency, as illustrated in FIG. 3, for the piston ring 40, during engine operation, to become seated on the sidewall surface 34 of the groove 32 nearest the combustion chamber 24 either because of a higher gas pressure on the opposite side of the piston ring 40 or for other reasons. A piston ring seated on the groove surface 34 has several undesirable effects on engine performance. First, there is an increased probability that oil will be scraped up by the piston ring 40 to the combustion chamber 24 resulting in higher oil consumption. Second, the increased heat flow from the combustion chamber 24 to the piston ring 40, and the resultant higher temperature gradient across the piston ring 40, sometimes causes the piston ring 40 to become twisted in its groove 32 as illustrated in FIG. 4. This twisting reduces the surface area contact between the piston ring 40 and the cylinder bore wall 22, which increases the leakage of both blow-by gas and lubricating oil 54 past the piston ring 40. A twisted piston ring 40 also undermines the seal between the piston ring 40 and the cylinder bore wall 22, and increases friction and wear on the cylinder bore wall surface 22.
Consequently, a need exists for an improved piston ring and groove design which reduces the possibility of the piston ring seating against the top surface of its groove during engine operation.